1. Field of the Invention
This invention relates to copper material having an aluminum film deposited thereon which is adapted to prevent copper atoms from diffusing and exuding therefrom by means of an extra-thin aluminum film deposited on a surface of the copper material. The aluminum film coated copper material is particularly useful as an electrically-conductive element or a thermally-conductive element.
2. Description of the Prior Art
It is well known in the art that copper has excellent thermal-conduction and electrical-conduction properties, and is extensively used in the manufacture of electrical wire and cable, because its resistivity is as low as 1.72 .mu..OMEGA.. cm at a normal temperature and it is produced in large amounts. Also, copper is used as a material for cooling devices, such as, for example, the cooling plate which contacts a heating element, an air-cooled cascade, an air-cooled plate, or the like, because of its high thermal conductivity which is next to silver.
However, copper is chemically reactive with sulfur (S) or the like. For example, if copper is placed in an atmosphere of hydrogen sulfide a film of copper sulfide forms on its surface. Also, when copper is left to stand in air for a long period of time, its surface becomes covered with patina. Further, copper has a disadvantage of being easily corroded by acid. In addition, copper is apt to diffuse. For example, when a copper substrate is used as a heat sink for a semiconductor device where a silicon substrate is mechanically placed in contact with copper substrate, copper atoms diffuse into the silicon substrate as a result of surface contact between both material. This has been a big problem in the semiconductor industry in which severe impurity control is required.
In an case of insulation wires where a copper conductor is covered with rubber, tin plating on the copper conductor is usually employed in view of the fact that copper that is easily corroded by sulfur, and also to prevent diffusion and exudation of copper atoms into the covering insulation material.
Aluminum is known as a material which prevents exudation of copper atoms without deteriorating the characteristics of copper, such as, for example, the thermal conductivity characteristics, the electrical-conductivity characteristics, and the like, and is commercially used as an aluminum cladded copper material which is made by covering a copper rod or wire with a cylindrical aluminum member. However, the conventional aluminum cladded copper material is unable to completely prevent the formation of natural oxide films, stain or the like which form on the surface of copper therefrom, because copper and aluminum are merely mechanically contacted together. Furthermore, aluminum cladding is disadvantageous in that it is difficult to decrease thickness of aluminum clad so that copper may exhibit its flexibility.